A printing apparatus having the function of a printer, copying machine, facsimile apparatus, or the like, or a printing apparatus used as an output device for a multifunction apparatus or workstation including a computer, word processor, or the like prints an image on a printing medium such as a printing sheet or thin plastic plate (used for an OHP sheet or the like) on the basis of image information.
Such printing apparatuses are classified by the printing method used into an inkjet type, wire dot type, thermal type, thermal transfer type, electrophotographic type, and the like.
Of these printing apparatuses, a printing apparatus of an inkjet type (to be referred to as an inkjet printing apparatus hereinafter) prints by discharging ink from a printhead onto a printing medium. The inkjet printing apparatus has many advantages: the apparatus can be easily downsized, print a high-resolution image at a high speed, and print on a plain sheet without requiring any special process. In addition, the running cost of the inkjet printing apparatus is low, and the inkjet printing apparatus hardly generates noise because of non-impact printing and can print a color image by using multicolor ink.
The inkjet printing method includes several methods, and one of the methods is a bubble-jet printing method in which a heater is mounted within a nozzle, bubbles are generated in ink by heat, and the foaming energy is used to discharge ink. A heating element which generates thermal energy for discharging ink can be manufactured by a semiconductor manufacturing process. Examples of a commercially available printhead utilizing the bubble-jet technique are (1) a printhead obtained by forming a heating element on a silicon substrate as a base to prepare a heating element substrate and joining to the element substrate a top plate which has a groove for forming an ink channel and is made of a resin (e.g., polysulfone), glass, or the like, and (2) a high-resolution printhead obtained by directly forming a nozzle on an element substrate by photolithography so as to eliminate any joint.
FIG. 13 is a circuit diagram showing an example of a heater driving circuit within a printhead mounted on an inkjet printing apparatus which prints by the bubble-jet printing method.
A heater (heating element) R1 formed on a printhead element substrate and a switching element Q1 for switching a current to the heater are series-connected between a power supply VH and ground. An arbitrary switching element is turned on/off in accordance with a control signal corresponding to printing information from the printing apparatus main body. Ink is discharged from a nozzle corresponding to the driven heater, forming an image.
In order to obtain a high-quality image in a printing apparatus having a printhead which discharges ink by utilizing thermal energy generated by the above-mentioned heater, the volume of each discharged ink droplet must always be stabilized at a constant value. For this purpose, it is considered desirable to keep the heat generation amount of the heater constant.
Letting V be the potential difference of the heater, R be the resistance value of the heater, and t be the voltage application time, a heat generation amount P of the heater which converts electric energy into thermal energy is given byP=(V2/R)·t  (1)
As is apparent from equation (1), the heat generation amount of the heater greatly changes depending on the resistance value of the heater and a voltage applied to the heater. The resistance value of the heater varies by about 20% owing to the manufacturing process of the heater. Several methods have been known as a method of suppressing the influence of such variations on the heat generation amount (see, e.g., U.S. Pat. Nos. 5,943,070 and 6,382,756).
According to the method disclosed in U.S. Pat. No. 5,943,070, the resistance value of a dummy heater which is formed in a printhead from the same material as that of a heater for ink discharge is measured, and the resistance value of the heater for ink discharge is calculated from the measured resistance value. The pulse width of a pulse signal to be supplied to the heater is adjusted in accordance with the calculated resistance value of the heater to optimize the heat generation amount of the heater.
According to the method disclosed in U.S. Pat. No. 6,382,756, the ON resistance of a switching element such as a MOS transistor which is series-connected to a heater also varies in the manufacture. The ON resistance of the MOS transistor is series-connected to the resistance of the heater between the power supply and ground. A voltage applied to the heater is a voltage divided at the ratio of the resistance of the heater to the ON resistance of the MOS transistor.
Hence, variations in the ON resistance of the MOS transistor are equivalent to changes in the term V of equation (1), and influence the heat generation amount of the heater. To suppress this influence, a dummy MOS transistor is formed in a printhead, similar to the method disclosed in U.S. Pat. No. 5,943,070. The ON resistance of the MOS transistor is measured, and the voltage V to be applied to the heater is calculated. By using the calculation result, the pulse width of a pulse to be supplied to the heater is so adjusted as to make the heat generation amount of the heater constant.
Under the above background, there has also been proposed to control a switching element so as to make the voltage between both ends of a heating element constant and supply a constant current to the heating element for the purpose of constant energy (see, e.g., U.S. Pat. No. 6,523,922).
Since the element substrate is made of a silicon substrate, not only a heating element is formed on an element substrate, but a driver for driving the heating element, a temperature sensor used to control the heating element in accordance with the temperature of the printhead, a driving controller for the driver, and the like may be formed on the element substrate.
FIG. 14 is a block diagram showing a representative example of the configuration of an element substrate for a conventional inkjet printhead (see U.S. Pat. No. 6,116,714).
As shown in FIG. 14, an element substrate 900 comprises a plurality of heating elements 901 which are parallel-arrayed and supply thermal energy for discharge to ink, power transistors (drivers) 902 which drive the heating elements 901, a shift register 904 which receives externally serially input image data and serial clocks synchronized with the image data, and receives image data for each line, a latch circuit 903 which latches image data of one line output from the shift register 904 in synchronism with a latch clock and parallel-transfers the image data to the power transistors 902, a plurality of AND gates 915 which are respectively arranged in correspondence with the power transistors 902 and supply output signals from the latch circuit 903 to the power transistors 902 in accordance with an external enable signal, and input terminals 905 to 912 which externally receive image data, various signals, and the like. Of these input terminals, the terminal 910 is a heating element driving GND terminal, and the terminal 911 is a heating element driving power supply terminal.
The element substrate 900 further comprises a sensor monitor 914 such as a temperature sensor for measuring the temperature of the element substrate 900, or a resistance monitor for measuring the resistance value of each heating element 901. A printhead in which a driver, a temperature sensor, a driving controller, and the like are integrated in an element substrate has already been commercially available, and contributes to improvement of the printhead reliability and downsizing of the apparatus.
In this configuration, image data input as serial signals are converted into parallel signals by the shift register 904, output to the latch circuit 903, and latched by it in synchronism with a latch clock. In this state, driving pulse signals for the heating elements 901 (enable signals for the AND gates 915) are input via an input terminal, and the power transistors 902 are turned on in accordance with the image data. A current then flows through corresponding heating elements 901, and ink in the liquid channels (nozzles) is heated and discharged as droplets from orifices at the distal ends of the nozzles.
FIG. 15 is a view showing in detail a part associated with variations in parasitic resistance on the element substrate for the inkjet printhead shown in FIG. 14.
A parasitic resistance (or constant voltage) component 916 which leads to a loss in supplying energy to the heating element upon application of a constant power supply voltage from the printing apparatus main body exists in the power transistor 902 (which is a bipolar transistor in this case, but may be a MOS transistor) shown in FIGS. 14 and 15, and a common power supply wiring line and GND wiring line for driving a plurality of heating elements. Further, in areas 2801 and 2802 encircled by broken lines as shown in FIG. 15, a voltage generated by the parasitic resistance 916 changes depending on the number of simultaneously driven heating elements 901, and as a result, energy applied to the heating element 901 varies.
The area 2801 contains a parasitic resistance component 2801a present in a power supply wiring line of the inkjet printing apparatus, a parasitic resistance component 2801b present in a power supply wiring line of the inkjet printhead, and a parasitic resistance component 2801c in a common power supply wiring line. Likewise, the area 2802 contains a parasitic resistance component 2802a present in a GND wiring line of the inkjet printing apparatus, a parasitic resistance component 2802b present in a GND wiring line of the inkjet printhead, and a parasitic resistance component 2802c in a common GND wiring line.
In practice, as shown in FIG. 15, the heating elements 901 serving as printing elements inevitably vary in absolute resistance value by ±20% to 30% in mass production owing to the difference in film thickness and its distribution in the substrate manufacturing process.
From this, a power transistor has been used as a driver for driving the heating element of an available inkjet printhead in order to mainly reduce the resistance. The power transistor 902 functions as a constant power supply having an opposite bias to a constant element driving power supply, or an ON resistance. Since a current flowing through the heating element 901 changes depending on variations in the resistance of the heating element, energy (power consumption) applied to the heating element during a predetermined time greatly changes depending on the resistance value of the heating element in the manufacture.
The energy change has conventionally been coped with by changing by the resistance of the heating element a pulse width applied to drive the heating element. With this measure, power consumption of the heating element is made constant so as to stably discharge ink by driving the inkjet printhead and achieve a long service life of the printhead.
In recent years, the number of necessary printing elements greatly rises for higher printing speed. At the same time, it becomes more necessary than a conventional printing apparatus to uniform energy applied to the heating element for higher printing resolution. As described above, as the difference in the number of simultaneously driven printing elements becomes larger, energy applied to the heating elements more greatly varies, and the service life of the printhead becomes shorter. This generates a fault such as degradation of the printing quality owing to energy variations.
As a recent technique, the driver part is so controlled as to supply a constant current to each heater in a configuration having an effect of making energy constant, as shown in FIG. 16. This configuration can solve the above-described problem because a constant current always flows through each heating element and energy, i.e., (resistance value of heating element)×(square of constant current value) is supplied regardless of the number of simultaneously driven printing elements unless the resistance value varies during use. A configuration which keeps a current flowing through the heating element constant has also been proposed (see, e.g., U.S. Pat. No. 6,523,922).
The heating element used for the printing element of the inkjet printhead is generally made of a material having a negative temperature coefficient (that is, the resistance of the heating element decreases along with temperature rise upon driving for discharge), as disclosed in Japanese Patent Laid-Open No. 56-89578 and U.S. Pat. No. 4,709,243.
In this case, if a current flowing through the heating element is so controlled as to make the voltage between both ends of the heating element constant, as disclosed in U.S. Pat. No. 6,523,922, the resistance of the heating element decreases at a high temperature, the current flowing through the heating element increases, and energy applied to the heating element further increases in the second half of driving.
It is known that the service life of the heating element becomes longer as the temperature of the heating element is lower after film boiling in bubble-jet printing utilizing the force of film boiling by heat of the heating element.
Among the printhead substrates, the resistance of the heating element (heating resistance element) which is the largest among resistance components varies by about 20% to 30% owing to manufacturing variations, as described above. Note that, in FIG. 16, the same reference numbers are added to the same constituent elements or matters as those described in FIGS. 14 and 15, and the description is omitted. Since the power supply voltage of the printing apparatus main body in a conventional mechanism is constant, energy applied to the heating element is made constant by adjusting a pulse width applied to the heating element upon variations in the resistance of the heating element, as also described above.
However, when a constant current is commonly supplied to the heating elements of a plurality of substrates in order to eliminate variations in energy caused by the difference in the number of simultaneously driven printing elements, like the prior art, the power loss on the inkjet printhead substrate by variations in the resistance of the heating element greatly changes.
FIG. 17 is a table showing variations in power loss when the heating element is driven at a constant current.
The example shown in FIG. 17 assumes variations in voltage generated at both ends of the heating element and manufacturing variations in heating element (in this case ±20%) when the resistance value of the heating element is about 100Ω and a 150-mA current is supplied as a constant current. FIG. 17 shows the ratio of energy consumed by constituent components other than the heating element when the heating element has a maximum resistance (120Ω), 1 V is necessary to control the driver voltage for a voltage (18 V) between both ends of the heating element, and a voltage (19 V) higher by 1 V is applied on the printing apparatus side in order to control a constant current. The power consumption of the heating element upon supply of a constant current changes (1.8 to 2.7 W) depending on variations (80 to 120Ω) in the resistance value of the heating element. Upon variations, application power is adjusted by changing the pulse width applied to the heating element in actual printing.
FIG. 17 also shows pulse widths necessary when energy is made constant.
In FIG. 17, as indicated in a dotted area 3001, when the resistance value of the heating element is 80Ω, about 58% of power applied to the heating element is mainly consumed (power loss) by a control part (driver part in the inkjet printhead substrate) for supplying a constant current. In order to make energy applied to the heating element constant even though the resistance value changes, the application pulse width is adjusted to 1.25 μs for a heating element resistance of 80Ω and 0.83 μs for a heating element resistance of 120Ω. As understood from a comparison between values in dotted areas 3002 and 3003, the ratio of these application pulse widths is about 1.5 times, and the difference in loss energy is different by about 10 times between the heating element resistances of 80Ω and 120Ω.
Particularly, when the resistance value of the heating element is 80Ω, about 58% of energy applied to the heating element is lost. On the other hand, when the resistance value of the heating element is 120Ω, the lost is about 6%. Thus, heat generated in the substrate also varies depending on the resistance value of the heating element.
If all the power is consumed within the inkjet printhead substrate, the substrate temperature goes up. This influences the ink discharge amount.
FIG. 18 is a graph showing the relationship between the printing time and the substrate temperature when a constant current is supplied to the inkjet printhead substrate.
As is apparent from FIG. 18, the degree of rise of the substrate temperature changes upon variations in the resistance of the heating element.
FIG. 19 is a graph showing the relationship between the ink temperature and the ink discharge amount.
As is apparent from FIG. 19, as the ink temperature changes, the ink discharge amount also changes. Since the ink temperature is influenced by the substrate temperature, the rise of the substrate temperature influences the ink discharge characteristic.
Hence, the fact that variations by about 20% to 30% in the resistance value of the heating element in manufacturing the printhead cannot be avoided means that it is very difficult to provide an inkjet printhead having uniform ink discharge performance.
As described above, when the method of driving the heating element at a constant current in order to eliminate the difference caused by a change in the number of simultaneously driven printing elements is introduced, energy is wastefully consumed owing to variations in the resistance value of the heating element in the printhead manufacturing process. Moreover, in actual printing, the temperature variation characteristic of the substrate changes, and the printing performance of the printhead greatly varies upon a change in ink viscosity or the like depending on the ink temperature.